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Thursday, 18 September 2008

Yellowstone National Park: The largest active volcano on earth

Excerpted from a wonderful book filled with interesting facts, authored by the brilliant Bill Bryson. I had no idea until I read the book that Yellowstone is the largest active volcano in the world, and that it is due to pop (approx 30,000 years overdue). When it goes, it will take out pretty much all of us (in the initial blast and thereafter, the 'volcanic winter'). Hope it doesn't happen within my lifetime... sheesh!

In the 1960s, while studying the volcanic history of Yellowstone National Park, Bob Christiansen of the United States Geological Survey became puzzled about something that, oddly, had not troubled anyone before: he couldn't find the park's volcano. It had been known for a long time that Yellowstone was volcanic in nature - that's what accounted for all its geysers and other steamy features - and the one thing about volcanoes is that they are generally pretty conspicuous. But Christiansen couldn't find the Yellowstone volcano anywhere. In particular, what he couldn't find was a structure known as a caldera.

Most of us, when we think of volcanoes, think of the classic cone shape of Fuji or a Kilimanjaro, which is created when erupting magma accumulates in a symmetrical mound. These can form remarkably quickly. In 1943 at Paricutin in Mexico a farmer was startled to see smoke rising from a patch on his land. In one week he was the bemused owner of a cone 152 metres high. Within two years it had topped out at almost 430 metres and was more than 800 metres across. Altogether there are some ten thousand of these intrusively visible volcanoes on Earth, all but a few hundred of them extinct. But there is a second, less celebrated type of volcano that doesn't involve mountain-building. These are volcanoes so explosive that they burst open in a single mighty rupture, leaving behind a vast subsided pit, the caldera (from a Latin word for cauldron). Yellowstone obviously was of this second type, but Christiansen couldn't find the caldera anywhere.

By coincidence, just at this time NASA decided to test some new high-altitude cameras by taking photographs of Yellowstone, copies of which a thoughtful official passed on to the park authorities on the assumption that they might make a nice display for one of the visitor centres. As soon as Christiansen saw the photos he realized why he had failed to spot the caldera: virtually the whole park - 9,000 square kilometres - was caldera. The explosion had left a crater nearly 65 kilometres across - much too huge to be perceived from anywhere at ground level. At some time in the past Yellowstone must have blown up with a violence far beyond the scale of anything known to humans.

Yellowstone, it turns out, is a supervolcano. It sits on top of an enormous hot spot, a reservoir of molten rock that begins at least 200 kilometres down in the Earth and rises to near the surface, forming what is known as a superplume. The heat from the hot spot is what powers all of Yellowstone's vents, geysers, hot springs and popping mud pots. Beneath the surface is a magma chamber that is about 72 kilometres across - roughly the same dimensions as the park - and about 13 kilometres thick at its thickest point. Imagine a pile of TNT about the size of an English county and reaching 13 kilometres into the sky, to about the height of the highest cirrus clouds, and you have some idea of what visitors to Yellowstone are shuffling around on top of. The pressure that such a pool of magma exerts on the crust above has lifted Yellowstone and its surrounding territory about half a kilometre higher than they would otherwise be. If it blew, the cataclysm is pretty well beyond imagining.

According to Professor Bill McGuire of University College London, 'you wouldn't be able to get within a thousand kilometres of it' while it was erupting. The consequences that followed would be even worse...

... Since its first known eruption 16.5 million years ago, it has blown up about a hundred times, but the most recent three eruptions are the ones that get written about. The last eruption was a thousand times as big as that of Mount St Helens [which 'exploded with the force of five hundred Hiroshima-sized atomic bombs, shooting out a murderous hot cloud at up to 1,050 kilometres an hour']; the ones before that was 280 times as big, and the one before that was so big nobody knows exactly how big it was. It was at least 2,500 times as big as St Helens, but perhaps 8,000 times as monstrous.

We have absolutely nothing to compare it to. The biggest blast in recent times was that of Krakatau in Indonesia in August 1883, which made a bang that reverberated around the world for nine days, and made water slosh as far away as the English Channel. But if you imagine the volume of ejected material from Krakatau as being about the size of a golf ball, then that from the biggest of the Yellowstone blasts would be the size of a sphere you could just about hide behind. On this scale, the Mount St Helens eruption would be no more than a pea.

The Yellowstone eruption of two million years ago put out enough ash to bury New York State to a depth of 20 metres or California to a depth of 6 metres. This was the ash that made Mike Voorhies' fossil beds in eastern Nebraska. That blast occurred in what is now Idaho, but over millions of years, at a rate of about 2.5 centimetres a year, the Earth's crust has travelled over it, so that today it is directly under northwest Wyoming. (The hot spot itself stays in one place, like an acetylene torch aimed at a ceiling.) In its wake it leaves the sort of rich volcanic plains that are ideal for growing potatoes, as Idaho's farmers long ago discovered. In another two million years, geologists like to joke, Yellowstone will be producing French fries for McDonald's and the people of Billings, Montana, will be stepping around the geysers.

The ash fall form the last Yellowstone eruption covered all or parts of nineteen western states (plus parts of Canada and Mexico) - nearly the whole of the United States west of the Mississippi. This, bear in mind, is the breadbasket of America, an area that produces roughly half the world's cereals. And ash, it is worth remembering, is not like a big snowfall that will melt in the spring. If you wanted to grow crops again, you would have to find some place to put all the ash. It took thousands of workers eight months to clear 1.8 billion tonnes of debris from the 6.5 hectares of the World Trade Center site in New York. Imagine what it would take to clear Kansas.

And that's not even to consider the climatic consequences. The last supervolcano eruption on Earth was at Toba, in northern Sumatra, 74,000 years ago. No-one knows quite how bit it was, but it was a whopper. Greenland ice cores show that the Toba blast was followed by six years of 'volcanic winter' and goodness knows how many poor growing seasons after that. The event, it is thought, may have carried humans right to the brink of extinction, reducing the global population to no more than a few thousand individuals. That would mean that all modern humans arose from a very small population base, which would explain our lack of genetic diversity. At all events, there is some evidence to suggest that for the next twenty thousand years the total number of people on Earth was never more than a few thousand at any time. That is, needless to say, a long time to spend recovering from a single blast.

All this was hypothetically interesting until 1973, when an odd occurrence made it suddenly momentous: water in Yellowstone Lake, in the heart of the park, began to run over the banks at the lake's southern end, flooding a meadow, while at the opposite end of the lake the water mysteriously flowed away. Geologists did a hasty survey and discovered that a large area of the park had developed an ominous bulge. This was lifting up one end of the lake and causing the water to run out at the other, as would happen if you lifted one side of a child's paddling pool. By 1984, the whole central region of the park - over 100 square kilometres - was more than a metre higher than it had been in 1924, when the park was last formally surveyed. Then, in 1985, the central part of the park subsided by 20 centimetres (about 9 inches). It now seems to be swelling again. The geologists realized that only one thing could cause this - a restless magma chamber. Yellowstone wasn't the site of an ancient supervolcano; it was the site of an active one. It was also at about this time that they were able to work out that the cycle of Yellowstone's eruptions averaged one massive blow every 600,000 years. The last one was 630,000 years ago. Yellowstone, it appears, is due.

... 'And how much warning would you get if Yellowstone was going to go?'He shrugged. 'Nobody was around the last time it blew, so nobody knows what the warning signs are. Probably you would have swarms of earthquakes and some surface uplift and possibly some changes in the pattern of behaviour of the geysers and steam vents, but nobody really knows.' 'So it could just blow without warning?' He nodded thoughtfully. The trouble, he explained, is that nearly all the things that would constitute warning signs already exist in some measure at Yellowstone. 'Earthquakes are generally a precursor of volcanic eruptions, but the park already has lots of earthquakes - twelve hundred and sixty of them last year. Most of them are too small to be felt, but they are earthquakes nonetheless.'

A change in the pattern of geyser eruptions might also be taken as a clue, he said, but these too vary unpredictably. Once the most famous geyser in the park was Excelsior Geyser. It used to erupt regularly and spectacularly to heights of 100 metres, but in 1888 it just stopped. Then in 1985 it erupted again, though only to a height of 25 metres. Steamboat Geyser is the biggest geyser in the world when it blows, shooting water 120 metres into the air, but the intervals between its eruptions have ranged from as little as four days to almost fifty years. 'If it blew today and again next week, that wouldn't tell us anything at all about what it might do the following week or the week after or twenty years from now,' Doss says. ' The whole park is so volatile that it's essentially impossible to draw conclusions from almost anything that happens.'